In the insights we find, among others, these answers to the question of why the Kohen Gadol does not Mekadesh Ishah on Yom Kippur:
"The RI HA'LAVAN suggests that the act of Chupah cannot be done on Yom Kipur for a different reason. Even if it is not a Shevus, there is another problem. There is no way to make a Chupah -- it cannot be done with Bi'ah because Bi'ah is forbidden on Yom Kipur; a normal walk-under Chupah cannot be done on Yom Kipur because the Kohen Gadol must remain sequestered and cannot leave the Azarah. (Similarly, the new wife cannot come to join him there for a Chupah, either because it would be a breach of his Perishah, or because of the ruling of Rashi (Kidushin 52b DH v'Chi) and the Tosefta (Erchin 2:1) that prohibit women from entering the Azarah when not bringing a Korban.)"
(a) The objections of the RI Halavan may be met as follows: The Kohen Gadol could do the Kidushin by Shaliah - and by a Shtar which had been prepared in advance on condition that it be neccesary
Perhaps, despite our knowledge that Erusin and Huppah make the woman an Esheth-Ish, nevertheless she is not "Beito" until the Kinus and that he cannot do during the Avoda, and a Shaliah can't do it for him. Yeshayahu HaKohen Hollander
"Rebbi Akiva Eiger adds that a Chupah cannot be performed at this point for an additional reason. The Gemara (Kesuvos 56a) discusses whether a Chupah which is not fit to be consummated with Bi'ah is a valid Chupah or not. The RAMBAM (Hilchos Ishus 10:2) rules that it is not a valid Chupah. Thus, a Chupah done on Yom Kipur is not valid since Bi'ah is forbidden on Yom Kipur, and the Chupah is not fit for Bi'ah. Even according to those who do not rule Halachically like the Rambam, and prohibit such a Chupah, our Gemara might be trying to avoid the issue of making such a Chupah because of the doubt cast on it in Kesuvos."
(b) From this it would seem that, according to the Rambam, those weddings at which the Huppa takes place in the afternoon are invalid because the Biah will not be done until after nightfall? Must the Bi'ah be on the same calendar day? If not - the Biah of the Kohen Gadol can be done at night, after a Seudath Mitzvah with Sheva Berahot as a fast-breaker.
(a) Doing the Kidushin via a Shali'ach will not help the Nisu'in (i.e. Chupah), which cannot be done with a Shali'ach (as Rebbi Akiva Eiger points out there). It is clear from the Sugya that she is not considered "Beiso" prior to the Chupah (see Tosfos Yevamos 13b, DH l'Rabos, as well).
(b) The fact that is not done during the day does not make the Chupah one which is unfit for Bi'ah, since there is no Isur d'Oraisa of Bi'ah during the day. It was the Rabanan who prohibited it in order to maintain high standards of Tzeni'us, and under certain cicumstances even they permitted it (Shabbos 86a). The Chupah, though, is certainly considered "Ra'uy l'Bi'ah."
-Mordecai